Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $238.95   12.602%   $269.0625   $269.06   $269.05   $269.10  
  Purchase #2 $142.83   4.860%   $149.7715   $149.77   $149.75   $149.80  
  Purchase #3 $55.81   6.862%   $59.6397   $59.64   $59.65   $59.60  
  Purchase #4 $178.89   4.167%   $186.3443   $186.34   $186.35   $186.30  
  Purchase #5 $21.53   8.499%   $23.3598   $23.36   $23.35   $23.40  
  Purchase #6 $153.99   11.358%   $171.4802   $171.48   $171.50   $171.50  
  Purchase #7 $109.48   12.461%   $123.1223   $123.12   $123.10   $123.10  
  7 purchase totals = $901.48       $982.7803   $982.77   $982.75   $982.80